Assuming the intent here is to use this UFS partition to share data between
Solaris and Gentoo, you are much better off creating the UFS filesystem
while booted in Solaris.

From Gentoo, you can mount that partition read-only (just enable UFS in
the kernel) and read off-of that partition. Well, that gives you kinda
"one-way" sharing [From Solaris to Gentoo only, not the other
way around.]. I do this, and it works very well for me.

If you are feeling adventerous, or the data you are sharing is
not important, you can mount the UFS partition read-write
from Gentoo, and hope for the best. Appreciate if you can
share your experiences here on the forum though.